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    <title>Linear Regression</title>
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    <script type="text/javascript" src="dygraph-combined.js"></script>
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    body { max-width: 640 px; }
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  <body>
    <h2>Linear Regression Demo</h2>
    <p>Click the buttons to generate linear regressions over either data
    series. If you zoom in and click the regression button, the regression will
    only be run over visible points. Zoom back out to see what the local
    regression looks like over the full data.</p>

    <div id="demodiv" style="width: 480px; height: 320px;"></div>

    <script type="text/javascript">
      var data = [];
      for (var i = 0; i < 120; i++) {
        data.push([i,
                   i / 5.0 + 10.0 * Math.sin(i / 3.0),
                   30.0 - i / 5.0 - 10.0 * Math.sin(i / 3.0 + 1.0)]);
      }
      var labels = ['X', 'Y1', 'Y2'];
      var orig_colors = [];

      g = new Dygraph(
              document.getElementById("demodiv"),
              data,
              {
                labels: labels,
                drawPoints: true,
                strokeWidth: 0.0,
                drawCallback: function(g, is_initial) {
                  if (!is_initial) return;
                  var c = g.getColors();
                  for (var i = 0; i < c.length; i++) orig_colors.push(c[i]);
                }
              }
          );

      // coefficients of regression for each series.
      // if coeffs = [ null, [1, 2], null ] then we draw a regression for series 1
      // only. The regression line is y = 1 + 2 * x.
      var coeffs = [ null, null, null ];
      function regression(series) {
        // Only run the regression over visible points.
        var range = g.xAxisRange();

        var sum_xy = 0.0, sum_x = 0.0, sum_y = 0.0, sum_x2 = 0.0, num = 0;
        for (var i = 0; i < data.length; i++) {
          var x = data[i][0];
          if (x < range[0] || x > range[1]) continue;

          var y = data[i][series];
          if (y == null) continue;
          if (y.length == 2) {
            // using fractions
            y = y[0] / y[1];
          }

          num++;
          sum_x += x;
          sum_y += y;
          sum_xy += x * y;
          sum_x2 += x * x;
        }

        var a = (sum_xy - sum_x * sum_y / num) / (sum_x2 - sum_x * sum_x / num);
        var b = (sum_y - a * sum_x) / num;

        coeffs[series] = [b, a];
        if (typeof(console) != 'undefined') {
          console.log("coeffs(" + series + "): [" + b + ", " + a + "]");
        }

        updateChart();
      }

      function updateChart() {
        // Generate a new data set with the regression lines.
        var new_labels = [];
        var new_colors = [];
        var new_opts = {};
        for (var i = 0; i < labels.length; i++) {
          new_labels.push(labels[i]);
          if (i) new_colors.push(orig_colors[i - 1]);
          if (coeffs[i]) {
            // Darken the series by 50% to generate its regression.
            var label = labels[i] + " Regression";
            new_labels.push(label);
            var c = new RGBColorParser(orig_colors[i - 1]);
            c.r = Math.floor(255 - 0.5 * (255 - c.r));
            c.g = Math.floor(255 - 0.5 * (255 - c.g));
            c.b = Math.floor(255 - 0.5 * (255 - c.b));
            new_colors.push(c.toHex());
            new_opts[label] = {
              drawPoints: false,
              strokeWidth: 1.0
            };
          }
        }

        var new_data = [];
        for (var i = 0; i < data.length; i++) {
          new_data.push([]);
          for (var j = 0; j < data[i].length; j++) {
            new_data[i].push(data[i][j]);
            if (coeffs[j]) {
              new_data[i].push(coeffs[j][0] + coeffs[j][1] * data[i][0]);
            }
          }
        }

        new_opts.file = new_data;
        new_opts.labels = new_labels;
        new_opts.colors = new_colors;
        g.updateOptions(new_opts);
      }

      function clearLines() {
        for (var i = 0; i < coeffs.length; i++) coeffs[i] = null;
        updateChart();
      }
      
    </script>
    
    <div style="position: absolute; left: 500px; top: 150px;">
    <input type=button style="color: green;" value="Regression (Y1)" onClick="regression(1)" />
    <br/><br/>
    <input type=button style="color: blue;" value="Regression (Y2)" onClick="regression(2)" />
    <br/><br/>
    <input type=button value="Clear Lines" onClick="clearLines()" />
    </div>
</body>
</html>
